Chapter 26 Digestive System

Metabolism and Digestive System Overview

Metabolism: Sum of all chemical reactions in the body.
- Catabolism: Breakdown of large molecules (e.g., proteins to amino acids).
- Anabolism: Building larger molecules (e.g., triglycerides from glycerol and fatty acids).
Digestive System Functions:
- Ingestion: Taking food into the mouth.
- Mechanical Processing: Physical breakdown of food (e.g., chewing).
- Digestion (Chemical): Chemical breakdown (e.g., proteins to amino acids).
- Secretion: Release of water, acids, enzymes, etc.
- Absorption: Movement of nutrients into the body.
- Excretion: Removal of waste.
  - Defecation: Bowel movement.
  - Feces: Waste material.
Protective Functions of GI Tract Lining:
- Mucous: Protects against acids and enzymes.
- Epithelial Regeneration: Protects against abrasion.
- Peyer's Patches: Protect against pathogens.

Peritoneum:
- Visceral Peritoneum: Covers abdominal organs.
- Parietal Peritoneum: Lines abdominal wall.
- Peritoneal Fluid: Lubricates.
Retroperitoneal Space: Space behind the parietal peritoneum; houses organs like kidneys.
Mesenteries: Double sheets of peritoneum that hold organs in place; contain blood vessels and nerves.
- Lesser Omentum: Connects stomach and liver.
- Falciform Ligament: Connects liver to anterior abdominal wall.
- Greater Omentum: Hangs from stomach, covers abdominal organs, accumulates fat.
Movement of Digestive Materials:
- Peristalsis: Waves of muscle contraction that move a bolus.
- Segmentation: Contraction of circular muscles that churn food.
Control of Digestive Function:
- Neural Mechanisms: Via enteric nervous system.
- Hormonal Mechanisms: Digestive hormones.
- Local Mechanisms: Chemicals released by digestive cells.

Functions:
- Sensory analysis, mechanical processing, lubrication, limited digestion.
Anatomy:
- Vestibule: Area between cheeks and teeth.
- Gingivae: Gums.
- Hard Palate: Maxilla and palatine bones.
- Soft Palate: Closes off nasopharynx during swallowing.
- Uvula: Prevents food from entering pharynx prematurely.
Tongue:
- Functions: Mechanical processing, manipulation of food, sensory analysis, secretion of mucous and lingual lipase.
- Lingual Frenulum: Connects tongue to floor of mouth.
Salivary Glands:
- Parotid: Secretes salivary amylase.
- Sublingual: Secretes mucous.
- Submandibular: Secretes mucous and salivary amylase.
- Saliva: Water, enzymes, buffers, mucins; lubricates, dissolves chemicals, begins digestion.
Teeth:
- Function: Mastication.
- Types: Incisors (cutting), cuspids (tearing), bicuspids/molars (grinding).
- Dental Succession:
  - Deciduous (baby) teeth: 20
  - Secondary (permanent) teeth: 32

Pharynx:
- Function: Passageway for food and air.
- Anatomy: Nasopharynx, oropharynx, laryngopharynx.
Esophagus:
- Function: Transports food to stomach.
- Swallowing (Deglutition) Phases:
  - Buccal: Bolus to oropharynx.
  - Pharyngeal: Swallowing reflex initiated.
  - Esophageal: Bolus enters esophagus.
- Peristaltic Waves: Move bolus down esophagus.
- Lower Esophageal Sphincter: Prevents reflux.

Functions:
- Food storage, mechanical breakdown, chemical breakdown, intrinsic factor production.
Anatomy:
- Lesser/Greater Curvature.
- Regions: Fundus, cardia, body, pyloris.
- Pyloric Sphincter: Controls movement into small intestine.
- Gastric Glands:
  - Parietal Cells: Secrete HCl.
  - Chief Cells: Secrete pepsinogen (converted to pepsin by HCl).
Regulation of Gastric Activity:
- Cephalic Phase: Triggered by sight/smell/taste of food.
- Gastric Phase: Food arrives in stomach.
- Intestinal Phase: Chyme enters small intestine.
Digestion and Absorption:
- Protein breakdown begins.
- Carb digestion stops.
- No nutrient absorption.

Functions:
- Digestion and absorption of nutrients; 90% of absorption occurs here.
Anatomy:
- Duodenum: Mixing bowl; receives chyme, secretions from pancreas and gall bladder.
- Jejunum: Majority of digestion and absorption.
- Ileum: Attaches to large intestine.
- Ileocecal Valve: Controls flow into large intestine.
- Internal Anatomy: Villi and microvilli increase surface area.
- Layers: Mucosa, submucosa, muscularis, serosa.
Gastric Reflexes:
- Gastroenteric: Food in stomach increases motility/secretion in small intestine.
- Gastrocolic: Food in stomach triggers defecation.

Pancreas:
- Functions: Secretes digestive enzymes and sodium bicarbonate; endocrine (insulin, glucagon) and exocrine organ.
- Anatomy: Head, body, tail; pancreatic duct empties into duodenum.
Liver:
- Functions: Metabolic regulation, hematologic regulation, bile production (emulsifier, not an enzyme).
- Anatomy: Hepatocytes, four lobes, fibrous capsule, falciform ligament, hepatic portal system.
  - Portal hypertension: Blocked blood vessels increase pressure, causing ascites.
- Bile Duct System: Right/Left hepatic ducts -> common hepatic duct -> (either common bile duct -> duodenum) OR (cystic duct -> gallbladder)
Gallbladder:
- Functions: Stores and concentrates bile; releases bile into duodenum in response to CCK.

Functions:
- Water reabsorption, waste compaction, vitamin absorption (K, biotin, pantothenic acid), fecal storage.
Anatomy:
- Cecum: First part, appendix extends from it.
- Colon: Ascending, transverse, descending, sigmoid.
  - Hepatic Flexure: Turn between ascending and transverse colon.
  - Splenic Flexure: Turn between transverse and descending colon.
- Rectum: Temporary storage of feces; expansion triggers defecation.
- Anus: Internal (involuntary) and external (voluntary) sphincters.
- MALT/Peyer’s Patches: Prevents bacteria from entering the small intestine and bloodstream.

Carbohydrates and Lipids: Digestion starts in mouth, resumes in small intestine (pancreatic amylase/lipase), bile emulsifies lipids.
Proteins: Digestion begins in stomach (pepsin/HCl), continues in small intestine (pancreatic proteases).
Water: Reabsorption in large intestine.
Vitamins and Minerals: Absorbed in small intestine; vitamins from bacteria in large intestine.
Ions: Absorbed in small and large intestine.